1. Field of the Invention
The present invention relates to a screw vacuum pump for use in combination with vacuum apparatus for creating a vacuum in industrial processes including vacuum packaging processes and semiconductor device manufacturing processes and, more particularly, to a vertical oilless screw vacuum pump for such purposes.
2. Description of the Prior Art
A known oilless screw vacuum pump shown in FIG. 7 has a set of meshing male and female screw rotors (hereinafter referred to simply as "rotors") 3 disposed within a rotor chamber formed in a casing 4 and opening at one end into a suction port 5 and at the other end into a discharge port 6. The rotors 3 are supported for rotation with a small gap therebetween each at the opposite ends thereof in bearings 1 and 2. Timing gears 7 are attached to the respective ends of the shafts of the rotors 3 on the side of the suction port 5, respectively, and the timing gears 7 are meshed. One of the timing gears 7 is in mesh with a driving gear 9 attached to the output shaft of a motor 14.
The rotation of the output shaft 8 is transmitted to the timing gear 7 at a speed increasing ratio to rotate the rotors 3. The rotate 3 rotates synchronously respectively in opposite directions to suck in a gas, compress the gas and discharge the compressed gas through the discharge port 6.
Generally, ultimate vacuum is the most the most significant value among those indicating the performance of the vacuum pump. With the screw vacuum pump shown in FIG. 7, ultimate vacuum is a minimum achievable vacuum in the rotor chamber when the screw vacuum pump is operated with the opening of the rotor chamber on the side of the suction port 5 closed.
In the foregoing known screw vacuum pump, the gears and the bearings are lubricated with a lubricating oil and hence the lubricating oil mixes with air. On the other hand, a vacuum state prevails in the vicinity of the suction port 5, while an atmospheric state prevails in the vicinity of the discharge port 6 and the gear chamber 12. Accordingly, air leaks from shaft seal parts 13 near the suction port 5 as indicated by an arrow a expressed by a broken line and reverse flow of air, as indicated by an arrow b expressed by a broken line, from the discharge port 6 toward the suction port 5 through the gaps between the teeth of rotors 3 increases to lower the ultimate vacuum. The volume of air at atmospheric pressure (760 torr) leaked into the rotor chamber 3 in a vacuum state of a vacuum in the range of up to 10.sup.-3 torr expands to a volume 10.sup.5 to 10.sup.6 times as much, and hence the rotors 3 are unable to function properly and the rotor chamber cannot be evacuated to a sufficiently high ultimate vacuum.
Furthermore, since the rotation of the motor 14 is transmitted at a speed increasing ratio through the gears 9 and 7 to the rotors 3, the screw vacuum pump must be provided with a comparatively large gear chamber 12 for containing the gears 7 and 9, which increases the size of the screw vacuum pump.